While organic electroluminescent (EL) devices have been known for over two decades, their performance limitations have represented a barrier to many desirable applications. In simplest form, an organic EL device is comprised of an anode for hole injection, a cathode for electron injection, and an organic medium sandwiched between these electrodes to support charge recombination that yields emission of light. These devices are also commonly referred to as organic light-emitting diodes, or OLEDs. Representative of earlier organic EL devices are Gurnee et al. U.S. Pat. No. 3,172,862, issued Mar. 9, 1965; Gurnee U.S. Pat. No. 3,173,050, issued Mar. 9, 1965; Dresner, “Double Injection Electroluminescence in Anthracene”, RCA Review, 30, 322, (1969); and Dresner U.S. Pat. No. 3,710,167, issued Jan. 9, 1973. The organic layers in these devices, usually composed of a polycyclic aromatic hydrocarbon, were very thick (much greater than 1 μm). Consequently, operating voltages were very high, often greater than 100V.
More recent organic EL devices include an organic EL element consisting of extremely thin layers (e.g. <1.0 μm) between the anode and the cathode. Herein, the term “organic EL element” encompasses the layers between the anode and cathode. Reducing the thickness lowered the resistance of the organic layers and has enabled devices that operate at much lower voltage. In a basic two-layer EL device structure, described first in U.S. Pat. No. 4,356,429, one organic layer of the EL element adjacent to the anode is specifically chosen to transport holes, and therefore is referred to as the hole-transporting layer, and the other organic layer is specifically chosen to transport electrons and is referred to as the electron-transporting layer. Recombination of the injected holes and electrons within the organic EL element results in efficient electroluminescence.
There have also been proposed three-layer organic EL devices that contain an organic light-emitting layer (LEL) between the hole-transporting layer and electron-transporting layer, such as that disclosed by C. Tang et al. (J. Applied Physics, Vol. 65, 3610 (1989)). The light-emitting layer commonly includes a host material doped with a guest material, otherwise known as a dopant. Still further, there has been proposed in U.S. Pat. No. 4,769,292 a four-layer EL element comprising a hole-injecting layer (HIL), a hole-transporting layer (HTL), a light-emitting layer (LEL) and an electron-transporting/injecting layer (ETL). These structures have resulted in improved device efficiency.
EL devices in recent years have expanded to include not only single color emitting devices, such as red, green and blue, but also white-devices, devices that emit white light. Efficient white light producing OLED devices are highly desirable in the industry and are considered as a low cost alternative for several applications such as paper-thin light sources, backlights in LCD displays, automotive dome lights, and office lighting. White light producing OLED devices should be bright, efficient, and generally have Commission International d'Eclairage (CIE) chromaticity coordinates of about (0.33, 0.33). In any event, in accordance with this disclosure, white light is that light which is perceived by a user as having a white color.
Since the early inventions, further improvements in device materials have resulted in improved performance in attributes such as color, stability, luminance efficiency and manufacturability, e.g., as disclosed in U.S. Pat. Nos. 5,061,569, 5,409,783, 5,554,450, 5,593,788, 5,683,823, 5,908,581, 5,928,802, 6,020,078, and 6,208,077, amongst others.
Notwithstanding all of these developments, there are continuing needs for organic EL device components, such as electron transporting materials and electron injecting materials, which will provide even lower device drive voltages and hence lower power consumption, while maintaining high luminance efficiencies and long lifetimes combined with high color purity.
A useful class of electron-transporting materials is that derived from metal chelated oxinoid compounds including chelates of oxine itself, also commonly referred to as 8-quinolinol or 8-hydroxyquinoline. Tris(8-quinolinolato)aluminum (III), also known as Alq or Alq3, and other metal and non-metal oxine chelates are well known in the art as electron-transporting materials. Tang et al., in U.S. Pat. No. 4,769,292 and VanSlyke et al., in U.S. Pat. No. 4,539,507 lower the drive voltage of the EL devices by teaching the use of Alq as an electron transport material in the luminescent layer or luminescent zone. Baldo et al., in U.S. Pat. No. 6,097,147 and Hung et al., in U.S. Pat. No. 6,172,459 teach the use of an organic electron-transporting layer adjacent to the cathode so that when electrons are injected from the cathode into the electron-transporting layer, the electrons traverse both the electron-transporting layer and the light-emitting layer.
Examples of electron-injecting layers include those described in U.S. Pat. Nos. 5,608,287, 5,776,622, 5,776,623, 6,137,223, and 6,140,763, the disclosures of which are incorporated herein by reference. An electron-injecting layer generally includes a material having a work function less than 4.0 eV. The definition of work function can be found in CRC Handbook of Chemistry and Physics, 70th Edition, 1989-1990, CRC Press Inc., page F-132 and a list of the work functions for various metals can be found on pages E-93 and E-94. Typical examples of such metals include Li, Na, K, Be, Mg, Ca, Sr, Ba, Y, La, Sm, Gd, Yb. A thin-film containing low work-function alkali metals or alkaline earth metals, such as Li, Cs, Ca, Mg can be employed for electron-injection. In addition, an organic material doped with these low work-function metals can also be used effectively as the electron-injecting layer. Examples are Li- or Cs-doped Alq.
U.S. Pat. Nos. 6,509,109 and 2003/0044643, the disclosures of which are incorporated herein by reference, describe an organic electroluminescent device wherein the electron injection region contains a nitrogen-free aromatic compound as a host material and a reducing dopant, such as an alkali metal compound. U.S. Pat. No. 6,396,209 describes an electron injection layer of an electron-transporting organic compound and an organic metal complex compound containing at least one alkali metal ion, alkali earth metal ion, or rare earth metal ion. Additional examples of organic lithium compounds in an electron-injection layer of an EL device include US 2006/0286405, US 2002/0086180, US 2004/0207318, U.S. Pat. No. 6,396,209, JP 2000-053957, WO 99/63023 and U.S. Pat. No. 6,468,676.
Fluoranthene derivatives are well known in the art as being useful as light-emitting compounds; for example, see US 2005/0271899, U.S. Pat. No. 6,613,454, US 2002/0168544, U.S. Pat. Nos. 7,183,010B2, 7,175,922, EP 1 718 124, EP 1 719 748, US 2006/0141287 and US 2007/0069198. US 2006/0238110 and WO 2007/039344 describe the use of polymeric fluoranthene derivatives as blue light-emitting dopants.
In particular, examples of 7,10-diaryl-fluoranthene derivatives as light-emitting compounds have been disclosed in JP 2002-069044, JP 2005-320286, US 2007/0069198, US 2005/0067955, US 2006/0246315, U.S. Pat. Nos. 6,803,120, 6,866,947, WO 2007/039344 and R. Tseng et al, Applied Physics Letters (2006), 88(9), 09351/1-3. Also, 3,8-Diphenylfluoranthene derivatives are disclosed as light emitters in US 2007/0063189.
US 2002/0022151 describes the use of 7,10-diaryl-fluoranthenes with at least one amino group directly substituted on the fluoranthene ring in light emitting layers as well as hole and electron transporting layers. US 2007/149815 describes the use of bis-aminofluoranthenes. US 2005/0244676 discloses the use of a 3-substituted fluoranthene derivatives with annulated rings in a light-emitting layer in combination with organic lithium salts in an electron-injecting layer.
The use of substituted fluoranthenes in an electron-transporting layer is also known; examples include devices described in US 2008/0007160, US 2007/0252516, US 2006/0257684, US 2006/0097227, and JP 2004-09144, the disclosures of which are incorporated herein by reference.
Substituted 1,10-phenanthroline compounds are also described as useful electron-transporting materials in JP 2003-115387; JP 2004-311184; JP 2001-267080; and WO 2002/043449. Additional phenanthroline compounds are reported in JP 2004-311184, JP 2004-175691, JP 2003-138251, JP 2003-123983, JP 2003-115387, EP 1 41 403, and WO 2004/026870. In particular, US 2007/0122657 describes devices containing a compound having a phenanthroline nucleus that is bonded to, or linked to, an aromatic system containing four or more fused aromatic rings and their use in electron-transporting layers.
WO 2007/126112 describes fluoranthenes having various fused ring aromatic substituents including phenanthroline substituents. General disclosures of phenanthrolines having a fluoranthene substituent have been made in EP 1 097 980 and WO 2004/026870. US 2006/0097227 describes phenanthroline derivatives and their use in electron-transporting and light-emitting layers of an EL device. Included in the disclosure are compounds containing a phenanthroline nucleus bonded to multiple fluoranthene substituents. However, these devices do not necessarily have all the desired EL characteristics in terms of high luminance in combination with low drive voltage.
Notwithstanding all these developments, there remains a need to improve efficiency and reduce drive voltage of OLED devices, as well as to provide embodiments with other improved features such as operational stability.